feat(nextjs): add connectivity check to example page

[shellmayr]

• Uses the new diagnoseSdkConnectivity() function to check whether something is preventing data from reaching sentry feat(browser): Add diagnoseSdkConnectivity() function to programmatically detect possible connectivity issues sentry-javascript#15821
• If the connection is disturbed, call this out to the user using a banner and link to the docs explaining how to solve the issue
• Contributes to TET-59

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After:

[github-actions]

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📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Generated by 🚫 dangerJS against 8025734

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 24.24%. Comparing base (e36505b) to head (8025734).
Report is 1 commits behind head on master.

Additional details and impacted files

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##           master     #951   +/-   ##
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  Coverage   24.24%   24.24%           
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  Files         121      121           
  Lines       12877    12877           
  Branches      713      713           
=======================================
  Hits         3122     3122           
  Misses       9734     9734           
  Partials       21       21           

Flag	Coverage Δ
unit-tests	24.24% <ø> (ø)

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[lforst]

beaut

(random thought: some applications put the Next.js devserver floaty thingy in the bottom right corner so idk whether the bottom right corner is a good place for the alert box. If it's easy I would put it in the center of the page with the other content but no biggie if that is too hard.)

[matejminar]

Looks good!
Nit: Can we slightly tweak the copy so that we take up only 3 lines? That one word in the fourth line makes it more massive than it needs to be.